Catheters have been used in the art for monitoring variations in blood pressure within a blood vessel, that is, within the cardiovascular system. Such catheters include those employing catheter tip transducers insertable into a blood vessel with the transducers providing direct pressure monitoring by transducing blood pressure at the region or regions of interest. Such catheter tip transducers may employ semiconductor material constructed and arranged with resistors and the like for use in developing an electrical signal representative of the monitored pressure and transmitting the signal by electrical conductors through the length of the catheter to monitoring systems located externally of the body of the subject being tested. Such monitoring systems usually display the subject's blood pressure wave form and also include a calibrated scale to give the pressure readouts in mm Hg. Typical monitoring systems include amplifiers, display devices and recording devices.
Examples of a single-sensor pressure measuring catheter are found in U.S. Pat. Nos. 4,274,423 and 4,722,348. The transducer disclosed in U.S. Pat. No. 4,274,423 includes a pressure sensor disposed within the end portion of a catheter. The pressure sensor takes the form of a pressure sensitive diaphragm constructed from a block of semiconductor material, such as silicon. The diaphragm is deflected in dependence upon the pressure and the deflection is sensed by one or more strain gauges located in the diaphragm. The strain gauges are connected by suitable conductors to a monitoring system which amplifies and displays the generated electrical signal. U.S. Pat. Nos. 4,809,704, 4,815,471 and 4,815,472 disclose multiple sensor pressure-measuring catheters with a plurality of pressure sensors spaced along the catheter.
Before a transducer such as those described above can be used, however, it must be calibrated with the monitoring system to ensure that the readings it produces are accurate. With prolonged use, transducers lose some sensitivity and become less accurate, thus making it necessary to calibrate the transducer to the monitoring system so that the monitoring system will be appropriately adjusted to compensate for such inaccuracy. Techniques have been devised in the prior art to ensure that the monitoring system reading corresponds to the transducer's sensitivity. Such techniques include electronic calibration procedures wherein the transducer end of the catheter is connected to an amplifier which contains circuitry which simulates transducer output voltages. A switch on the amplifier is activated and a voltage is produced which should be equivalent to the voltage produced by the transducer under ideal conditions. The disadvantage of this technique is that it does not take into account mechanical changes to the transducer, such as wear of the silicon diaphragm resulting from repeated use and protein build up resulting from insertion of the transducer into the living body. Such mechanical changes can result in inaccurate calibration of the transducer.
A second technique for calibration uses a syringe connected to one end of a section of tee-shaped tubing to provide an airtight chamber. The transducer end of the catheter is inserted into a second end of the tee and the output hose of a mercury manometer is inserted into the third end of the tee. A plunger is extended into the syringe barrel until the manometer indicates an appropriate calibration pressure, such as 100 mm Hg. Because of the tee connection, the transducer diaphragm is also subjected to the same pressure. Most likely, the monitoring system will indicate some pressure differing from that measured by the manometer due to the inaccuracy of the transducer. The monitoring system's sensitivity control must therefore be adjusted until it gives an accurate pressure reading. This procedure is repeated over a range of pressures to check linearity. Unfortunately, this method of calibration is susceptible to errors resulting from temperature effects and difficulty in sustaining a constant accurate pressure using the syringe.
It is, therefore, a principal object of the present invention to provide a novel apparatus and method for accomplishing accurate, repeatable calibrations of catheter tip pressure transducers while minimizing errors resulting from pressure leaks, temperature effects and mechanical changes to the transducer.
It is a feature of this invention to provide a novel system for calibration of single-sensor and dual-sensor catheter tip pressure transducers.
It is a further feature of this invention to provide a novel system for calibrating a plurality of catheter tip pressure transducers.
Other and further objects and features of the invention will become obvious to one skilled in the art upon an understanding of the illustrative embodiment about to be described and various advantages, not referred to herein, will occur to one skilled in the art upon employment of the invention in practice.